Acquisition of the H19 methylation imprint occurs differentially on the parental alleles during spermatogenesis

The imprinted mouse H19 gene is hypomethylated on the expressed maternal al lele and hypermethylated on the silent paternal allele. A 2-kb region of di fferential methylation located from -2 to -4 kb relative to the H19 transcr iptional start site has been proposed to act as the imprinting mark since h ypermethylation in this region is inherited from sperm and retained on the paternal allele throughout development. Here, we describe a temporal analys is of the methylation patterns at the H19 locus during postnatal male germ cell development. The 2-kb region is methylated on the paternal allele thro ughout spermatogenesis, suggesting that methylation is acquired in this reg ion prior to the resumption of mitosis in postnatal male mice. Likewise, mo re than half of the maternal alleles are hypermethylated prior to the resum ption of mitosis. However, the remaining maternal alleles are not hypermeth ylated until the completion of meiosis I, indicating that de novo methylati on in this region is a continuous process. Sequences proximal to the H19 pr omoter, which are methylated in spermatozoa and on the paternal allele in s omatic cells, are differentially methylated in diploid, mitotic spermatogon ia. The maternal allele becomes hypermethylated in this region during meiot ic prophase. Thus, the parental H19 alleles acquire methylation differentia lly in the male germline. (C) 1999 Academic Press.